This invention relates generally to a system and method for tank pressure compensation and specifically to a system and method for fuel tank pressure compensation for an internal combustion engine and, more particularly, this invention relates to a barrier tank assembly utilized to reduce diurnal emissions from a fuel tank, particularly in a marine vessel.
Vehicles powered by internal combustion engines have at least one fuel tank that generally holds a supply of liquid fuel for the engine. The tanks are typically connected to a filler tube that is used to introduce fuel into the tank. The outer opening of the filler tube is usually covered with a removable cap.
When fuel is added to the tank, it displaces the air in the tank. The air, which is laden with fuel vapor, rushes out of the tank as the fuel enters. In many situations, foam is created by agitation of the fuel entering the tank. In some vehicles, the displaced air and foam rushes back to the filler tube as the tank is filled and splashes out on the person filling the tank. Other fuel systems include a vent line that extends from the interior of the tank to the atmosphere. The vent line enables air to escape from the tank as it is filled with fuel through the filler tube. The vent line also enables air to enter the tank as fuel is withdrawn for delivery to the engine.
The fuel tank vent line also serves to prevent pressure from building in the tank. If the tank were un-vented, increasing temperature of the fuel would cause fuel and vapor expansion that would cause the pressure in the tank to rise. If the pressure became too high, the fuel tank could rupture, causing fire or explosion.
Fuel systems used on marine crafts usually include a vent line from the fuel tank. The vent line typically opens to the atmosphere over the water. As the fuel tank is filled to near the top, the air flowing out of the vent line can carry fuel and foam overboard on to the water. Wave action that rocks a boat can also cause fuel to be discharged overboard both during fueling and when the tank is full. In addition, thermal expansion of the fuel due to an increase in fuel temperature may also cause either or both fuel and fuel vapor to be discharged overboard when the tank is full.
Thermal expansion refers to the expansion of fuel when it is heated to a higher temperature. Both gasoline and diesel fuel expand when their temperature rises. For example, fifty gallons of gasoline will expand by approximately 1.61 gallons when the temperature of the gasoline increases by thirty-four degrees Celsius. Similarly, two hundred gallons of gasoline will expand by approximately 6.46 gallons when the temperature of the gasoline is raised by thirty-four degrees Celsius. Diesel fuel expands at a lower rate than gasoline. For example, fifty gallons of diesel fuel will expand by approximately 1.36 gallons and two hundred gallons of diesel fuel will expand by approximately 5.44 gallons when the temperature of the diesel fuel is raised by thirty-four degrees Celsius. Thermal expansion can cause fuel to expand and fuel vapor to be forcibly discharged overboard via the vent line when the fuel tank does not have the space to accommodate the excess fuel and fuel vapor. Fuel and vapor discharged overboard poses a pollution hazard and is harmful to wildlife. There is also a risk that fuel floating on the water or emitted fuel vapor may catch fire causing injury to life or property. Furthermore, when fuel in the fuel tank is consumed and/or cooled, the volume is reduced. Air is drawn into the fuel tank through the vent line and becomes saturated with fuel vapor. Conversely, when this fuel in the tank is then warmed or is filled with additional fuel, the fuel expands and fuel vapor is forced out the vent line.
Accordingly, what is needed is a system and method to allow for some expansion and contraction without inducing air into the fuel tank or fuel vapor to the atmosphere.